Th- e pineal gland (also called the pineal body, epiphysis cerebri, epiphysis,conarium - or the "third eye") is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative me- latonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions.Its shape resembles a tiny pine cone(hence its name), and it is located near the centre of the brain, between the twohemispheres, tucked in a groove where the two rounded thalamic bodies join.

Location

The pineal gland is reddish-gray and about the size of a grain of rice (5–8 mm) in humans, located just rostro-dorsal to the superior colliculus and behind and beneath thestria medullaris, between the laterally positioned thalamic bodies. It is part of theepithalamus.

The pineal gland is a midline structure (the geometrical center) shaped like a pine cone,

[3] and is often seen in plain skull X-rays, as it is often calcified.[4]

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[e- dit]Miscellaneous anatomy

Pinealocytes in many non-mammalian vertebrates ha- ve a strong resemblance to the photoreceptor cellsof the eye. Some evolutionary biologists believe that the vertebrate pineal cells share a commonevolutionary ancestor with retinal cells.[11]

In some vertebrates, exposure to light can set off a chain reaction of enzymatic events within the pineal gland that regulate circadian rhythms.[12] Some early vertebrate fossil skulls have a pineal foramen(opening). This correlates with the physiology of the modern "living fossils," the lamprey and the tuatara, and some other vertebrates that have a parietal organ or "third eye," which, in some of them, is photosensitive. The third eye represents evolution's earlier approach to photoreception.[13] The structures of the third eye in the tuatara are analogous to the cornea, lens and retina, though the latter resembles that of an octopus rather than a vertebrate retina. The asymmetrical whole consists of the "eye" to the left and the pineal sac to the right. "In animals that have lost the parietal eye, including mammals, the pineal sac is retained and condensed into the form of the pineal gland."[13]

Unlike much of the rest of the mammalian brain, the pineal gland is not isolated from the body by the blood–brain barrier system;[14] it has profuse blood flow, second only to the kidney.

Fossils seldom preserve soft anatomy. The brain of the Russian Melovatka bird, about 90 million years old, is an exception, and it shows a larger-than-expected parietal eye and pineal gland.[15]

In humans and other mammals, the light signals necessary to set circadian rhythms are sent from the eye through theretinohypothalamic system to the suprachiasmatic nuclei (SCN) and the pineal.

[edit]Function

The pineal gland was originally believed to be a "vestigial remnant" of a larger organ. In 1917 it was known that extract of cow pineals lightened frog skin. Dermatology professor Aaron B. Lerner and colleagues at Yale University, hoping that a substance from the pineal might be useful in treating skin diseases, isolated and named the hormone melatonin in 1958.[16] The substance did not prove to be helpful as intended, but its discovery helped solve several mysteries such as why removing the rat's pineal accelerated ovary growth, why keeping rats in constant light decreased the weight of their pineals, and why pinealectomy and constant light affect ovary growth to an equal extent; this knowledge gave a boost to the then new field of chronobiology.[17]

Melatoni- n is N-acetyl-5-methoxy-tryptamine,- a derivative of the amino acid tryptophan, which also has other functions in the central nervous system. The production of melatonin by the pineal gland is stimulated by darkness and inhibited by light.[18] Photosensitive cells in the retina detect light and directly signal the SCN, entraining its rhythm to the 24-hour cycle in nature. Fibers project from the SCN to the paraventricular nuclei (PVN), which relay the circadian signals to the spinal cord and out via the sympathetic system tosuperior cervical ganglia (SCG), and from there into the pineal gland.

The compound pinoline is also produced in the pineal gland; it is one of the beta-carbolines.[citation- needed]

The human pineal gland grows in size until about 1–2 years of age, remaining stable thereafter,[19][20] although its weight increases gradually from puberty onwards.[21][22] The abundant melatonin levels in children are believed to inhibit sexual development, and pineal tumors have been linked with precocious puberty. When puberty arrives, melatonin production is reduced.

Calcification of the pineal gland is typical in adults, and has been observed in children as young as 2. Calcification rates vary widely by country and tend to increase by age, with calcification occurring in an estimated 40% of Americans by their 17th year.[4] Calcification of the pineal gland is largely associated with corpora arenacea (or "acervuli," or "brain sand").

Apparently the internal secretions of the pineal gland inhibit the development of the reproductive glands, because in cases where it is severely damaged in children, the result is accelerated development of the sexual organs and the skeleton.[23] In animals, the pineal gland appears to play a major role in sexual development, hibernation, metabolism, and seasonal breeding.[24]

Pineal cytostruc- ture seems to have evolutionary similarities to the retinal cells of chordates.[11] Modern birds- and reptiles have been found to express the phototransducing pigment- melanopsin in the pineal gland. Avian pineal glands are believed to act like the SCN inmammals.[25]

Studie- s on rodents suggest that the pineal gland may influence the actions of recreational drugs, such as cocaine,[26] andantidepressan- ts, such as fluoxetine (Prozac),[27] and its hormone melatonin can protect against neurodegeneration.[28- ]

Some recent studies show that the degree of pineal gland calcification is significantly higher in Alzheimer's disease patients vs. other types of dementia.[29]

Pineal gland calcification may also contribute to the pathogenesis of Alzheimer's disease and may reflect an absence of crystallization inhibitors.[30]

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[edit]M- ysticism, metaphysics and philosophy

The secretory activity of the pineal gland is only partially understood. Historically, its location deep in the brain suggested to philosophers that it possessed particular importance. This combination led to its being regarded as a "mystery" gland with mystical,metaphysical a- nd occult theories surrounding its perceived functions.

René Descartes, dedicating much time to the study of the pineal (Pine-cone shaped) gland, has called it the "principal seat of thesoul."[33] He believed that it was the point of connection between the intellect and the body.[34] Descartes attached significance to the gland because he believed it to be the only section of the brain which existed as a single part, rather than one half of a pair. He argued that because a person can never have "more than one thought at a time," external stimuli must be united within the brain before being considered by the soul, and he considered the pineal gland to be situated in "the most suitable possible place for this purpose," located centrally in the brain and surrounded by branches of the carotid arteries.[33]

Baruch de Spinoza criticized Descartes' viewpoint for neither following from self-evident premises nor being "clearly and distinctly perceived" (Descartes having previously asserted that he could not draw conclusions of this sort), and questioned what Descartes meant by talking of "the union of the mind and the body."[35]

The notion of a "pineal-eye" is central to the philosophy of the French writer Georges Bataille, which is analyzed at length by literary scholar Denis Hollier in his study Against Architecture. In this work Hollier discusses how Bataille uses the concept of a "pineal-eye" as a reference to a blind-spot in Western rationality, and an organ of excess and delirium.[36] This conceptual device is explicit in his surrealist texts, The Jesuve and The Pineal Eye.[37]

Numerous spiritual philosophies contain the notion of an inner Third Eye that is related to the ajna chakra and also the pineal gland, and to which is attributed significance in mystical awakening or enlightenment, clairvoyant perception and higher states of consciousness. This idea occurs historically in ancient, central and east Asia; and also in contemporary metaphysical theories relating to yoga,Theosophy, Pagan re- ligions, and New Age spiritual philosophies.